The present invention relates generally to optical fiber connection devices, and more particularly to embedding such optical fiber connection devices within a composite structure.
Composite structures are being employed in an increasing number of applications, such as a variety of automotive and aviation applications. An emerging area of interest with respect to composite structures involves the design and development of xe2x80x9csmart structures.xe2x80x9d Smart structures generally refer to composite structures which include one or more interactive optical or electronic devices. For example, fiber optic sensors can be used to measure a wide array of structural and environmental conditions relevant to composite structural integrity. These measurements include such parameters as strain, temperature, pressure, acceleration, acoustic emission and moisture content present within the composite structure.
These sensors must be able to receive, and oftentimes transmit, signals in order to function as desired. These signals can be transmitted via optical fibers embedded in the composite structure. However, widespread use of embedded fiber optic sensors has been hampered in large part due to the lack of a robust, reliable method for egressing the embedded fiber from the composite structure or for otherwise establishing optical communication with the embedded fiber. In other words, difficulty has been encountered connecting an external optical fiber lead with the embedded fiber and optical sensor.
Conventional continuous egress methods address this problem by running or extending a continuous fiber beyond the edge of the structure during layup of the composite plies. Once cured, the composite structure contains a fiber optic lead extending beyond the edge. Oftentimes, it is desirable to trim the edge of the composite structure to properly size the composite structure and to better finish the edge surface. Difficulties arise from this method in that the structure cannot be trimmed without cutting the fiber optic lead. In addition, even if the structure remains untrimmed, the fiber is vulnerable at the point of egress and may be accidentally broken or otherwise damaged. This is especially true when the structure is being debagged or otherwise handled prior to installation. If the fiber is broken or cut, there is no easy method of reconnecting to the embedded fiber and the structure may have to be scrapped.
A variation on the edge egress methods involves egressing the optical fiber out of the top or bottom surface of the composite structure. In this case, the structure edges may be trimmed without interference from the optical fiber. However, the optical fiber is still prone to being fractured or cut while the composite part is handled during fabrication or otherwise.
In order to avoid egress of an optical fiber beyond the composite structure, a fiber optic connector could be mounted upon the optical fiber and egressed from or embedded within the composite structure. However, conventional fiber optic connectors are not effectively embeddable because they are too large and would compromise the structural integrity of the composite structure. This problem is addressed by U.S. Pat. No. 6,035,084 to Haake et al., which discloses an optical fiber extended through a microtube which is embedded in the composite. The small diameter of the microtube lessens the risk of compromising the structural integrity of the structure. However, proper alignment of the embedded optical fiber with an external optical fiber requires the use of a lens, such as a graded index (GRIN) lens. Once the external optical fiber is properly aligned using the lens, the lens and external optical fiber are bonded in place. If the external optical fiber is removed or accidentally broken, the entire alignment and bonding procedure must generally be repeated to reestablish the connection.
It would be therefore advantageous to have an improved technique for establishing and maintaining optical communications with an embedded optical fiber. In this regard, it would be desirable to establish and maintain optical communications with an embedded optical fiber in a manner that has minimal impact on the structural integrity of the composite structure, will be unlikely to be disturbed during part handling and is straightforward to utilize.
Embedded fiber optic sensors can be used to measure a wide array of structural conditions in structural parts including, but not limited to, strain, temperature, pressure, acceleration, acoustic emission and moisture content. A simple, robust technique for accessing the embedded sensors is crucial to enable widespread use of embedded fiber optic sensors in aerospace structures and other commercial applications including pressure vessels, ship and submarine structures, and manufacturing machines.
According to one aspect of the present invention a fiber optic connector adapted to be embedded within a composite structure is provided. The connector comprises a sheath, an alignment sleeve and an internal connector ferrule. The sheath is embedded within the composite structure and has an opposed first and second ends. The first end of the sheath is flush with a surface of the composite structure. The alignment sleeve is disposed within the sheath. The internal connector ferrule is mounted upon an optical fiber, and is snugly disposed within the alignment sleeve proximate the second end of the sheath. The flush arrangement of the sheath advantageously protects the connector from damage during handling or use of the composite structure.
According to another aspect of the present invention, a composite structure is provided that includes a fiber optic connector as described above that is embedded within a plurality of layers of composite laminate. At least one optical fiber is also embedded within the layers and is operable to transmit a signal through those layers and the composite structure. The optical fiber is attached at one end to the fiber optic connector and at the other end to one or more sensors, such as strain sensors. As such, the fiber optic connector facilitates reliable optical communication with the embedded sensor.
The current invention also includes a range of other aspects. The sheath can have an outside diameter of less than 0.1 inches to minimize its impact on the structural integrity of the composite structure in which it is embedded. The alignment sleeve and the internal connector ferrule can cooperate to concentrically align the optical fiber within the alignment sleeve to within 0.00004 inches for an improved optical fiber connection. The fiber optic connector can resist cure temperatures of at least 350xc2x0 F. by having a sheath constructed of aluminum and the alignment sleeve and connector ferrule constructed of ceramic.
In order to communicate with the embedded sensor, an external ferrule mounted on an external optical fiber is inserted into the connector. The external connector ferrule is inserted into the connector and snugly disposed within the alignment sleeve proximate the first end of the sheath. In this arrangement, the ferrules are concentrically aligned, thereby placing the internal and external optical fibers in optical communication. This arrangement is desirable, for example, when an embedded sensor connected to the internal optical fiber must be interrogated by an analyzer connected to the external optical fiber. Disconnection and reconnection may be easily accomplished simply by removing and reinserting the external ferrule into the connector as desired.